1. Field of the Invention
The present invention relates to a semiconductor memory. More particularly, the present invention relates to a magnetic tunneling junction (MTJ) cell having a free magnetic layer with a low magnetic moment and a magnetic random access memory (MRAM) having the same.
2. Description of the Related Art
A magnetic random access memory (MRAM) consists of a transistor and a magnetic tunneling junction (MTJ) cell in which data is stored. A magnetoresistance (MR) ratio of an MTJ cell changes depending on magnetic polarization orientations of vertically stacked magnetic layers. The MRAM is a memory device that writes data using the characteristics of the MTJ cell.
In order to accurately read out data from an MRAM, it is preferable that the MRAM has as large a sensing margin as possible. The sensing margin is generally determined by the MR ratio of the MTJ cell.
In order to increase the MR ratio of the MTJ cell, stability and uniformity of the MTJ cell must be assured. For this purpose, a tunneling oxide layer of the MTJ cell must have a uniform thickness and stability of manufacturing processes must first be established.
In addition, the MRAM must assure selectivity, i.e., other MTJ cells adjacent to a selected MTJ cell must not be influenced during an operation of selecting the selected MTJ cell. In a well-known MRAM (hereinafter, referred to as “the conventional MRAM”), however, abnormal phenomena, such as an edge pinning and a vortex pinning, have been reported during writing or reading data to or from an MTJ cell.
Due to these phenomena, larger switching fields are demanded during the data read/write operation. This results in failed bits, i.e., MTJ cells which are not switched by a normal switching field during the data read/write operation.
FIG. 1 illustrates an edge pinning that occurs in a free magnetic layer during the data read/write operation of the conventional MTJ cell, and FIG. 2 illustrates a variation in hysteresis characteristics arising from the edge pinning.
FIG. 3 illustrates a vortex pinning that occurs in a free magnetic layer during the data read/write operation of the conventional MTJ cell, and FIG. 4 illustrates a variation in hysteresis characteristics arising from the vortex pinning.
In FIGS. 2 and 4, a reference symbol “▪” indicates a hysteresis characteristic when a magnetic field is applied, and a reference symbol “◯” indicates a hysteresis characteristic when no magnetic field is applied.
FIG. 5 illustrates a switching distribution of MTJ cells during the data write operation of the conventional MRAM, in which free magnetic layer is formed of CoFe/NiFe, here, a cobalt iron (CoFe) layer having a thickness of 10 Å and a nickel iron (NiFe) layer having a thickness of 30 Å. In FIG. 5, reference symbols C1, C2, C3 and C4 indicate ratios of failed bits, i.e., MTJ cells that are not switched in a normal switching field.
Referring to FIG. 5, the MTJ cells that are not switched in the normal switching field are switched in a switching field that is well above the normal switching field (C1, C2, C4), or are switched in a switching field that is well below the normal switching field (C3).
In the conventional MRAM, the degree of integration can be increased by forming a thinner magnetic layer of the MTJ cell, especially a free magnetic layer (CoFe/NiFe). As the free magnetic layer is thinner, however, thermal stability in the magnetic moment of the free magnetic layer may be degraded. This means that the MTJ cell is thermally unstable.